The widespread utilization of composite materials such as graphite and KEVLAR.RTM. (KEVLAR is a registered trademark of E.I. du Pont de Nemours & Co., Wilmington, Del. for an aromatic polyamide fiber of high tensile strength) embedded in a resinous matrix, e.g., epoxy, has become commonplace in the aerospace industry due to the high strength-to-weight ratio of such composite materials. Even though composite materials are formed into composite articles in the main by various molding processes, e.g., resin transfer molding, vacuum bag molding using prepreg composite materials, composite articles are generally subjected to standard post-cure machining practices such as drilling, routing, milling, and/or diamond wheel cutting to form the composite articles to net shape and/or to prepare the composite articles for integration with other articles.
Due to the brittle nature of some of the constituents elements used to form composite materials, e.g., graphite or KEVLAR.RTM. fibers, post-cure machining of composite articles often results in the generation of particulate debris, e.g., relatively fine particles less than 5 microns in size down to sub-micron sizes, i.e., dust, as well as larger fiber particles, i.e., greater than 5 microns in size up to several millimeters in size. Such particulate debris raises health concerns, i.e., respiration potential, for personnel working in areas where post-curing machining of composite articles is being undertaken as well concerns vis-a-vis contamination of post-cure machining tools, bearings, spindles, control electronics associated with automated machining tools, and aircraft avionics. The problems associated with the control and disposal of particulate debris arising from post-cure machining of composite articles differs from the problems arising out of the machining of metallic articles, which are generally more ductile than composite materials, thereby tending to form larger-sized particulate debris which is more readily controlled and disposed of.
Several techniques are currently used to alleviate the problems arising as a result of the particulate debris generated during the post-cure machining of composite articles. These techniques include coolant flooding, localized vacuum dust removal, and downdraft benches/booths.
In the coolant flooding technique, a specialized liquid, typically water containing anti-corrosive additives, is directed at the composite article during the machining process to entrain the generated particulate debris in solution. The coolant flooding technique is advantageous inasmuch as the coolant fluid lowers the operating temperature of the machining tool, thereby extending the useful lifetime of the fool. One disadvantage of this technique is the relatively high risk of composite article contamination. If the composite article includes stiffeners such as cores or foam, such stiffeners are subject to contamination if subjected to the coolant fluid. Vacuum fixtures used to hold the composite article during the machining process may draw the coolant fluid into contact with porous plies comprising the composite article. In addition to the foregoing disadvantage, the particulate debris solution produced as a result of the coolant flooding technique must be treated prior to disposal, typically by filtering using a cartridge-type filter or a paper roller filter, either of which require a high level of maintenance. Even with coolant flooding, a residual film of particulate debris usually remains on the composite article. This residue must be removed from the composite article using air pressure and absorbent materials. Finally, the coolant flooding technique obstructs the view of the composite article during the machining process to a degree. Overall, the coolant flooding technique is relatively costly and inflexible.
The localized vacuum debris removal technique utilizes hoses and a pickup head proximal the machine tooling or composite workpiece to create a localized aerodynamic fluid flow field that directs particulate debris into the pickup head for subsequent disposal. The localized vacuum debris removal technique does not utilize liquid coolants, thereby eliminating the possibility of workpiece contamination. The vacuum hoses and pickup head, however, are mounted directly in combination with the machining tool, and such a mounting arrangement is cumbersome and often obstructs the operator's view of the machining tool. Furthermore, this type of mounting arrangement may not be conducive to post-cure machining of composite articles having intricate or complex shapes. In addition, the localized vacuum debris removal technique is relatively inefficient over the wide size range of particulate debris inasmuch as the size of the hose and pickup head are limited due to the requirement to mount these components in combination with the tool.
In the downdraft bench/booth technique, the workpiece to be postcure machined is mounted in the bench/booth so that a localized aerodynamic fluid flow field is directed down over the workpiece such that particulate debris generated during machining is directed downwardly away from the workpiece. This technique is not subject to ply contamination, and does not require that attachment of bulky components directly in combination with the machining tool. The downdraft technique, however, is less efficient than the localized vacuum dust removal technique such that there is a higher risk that a machine operator will be exposed to particulate debris. Moreover, this technique is even less conducive than the localized vacuum dust removal technique to post-cure machining of composite articles having intricate or complex shapes.
The use of electrostatic technology for particle control is well known in the prior art. Representative examples of the use of such electrostatic technology includes U.S. Pat. Nos. 5,215,558, 5,125,124, 4,941,224, 4,715,870, 4,662,903, 4,509,958, 4,248,162, 4,147,522, 4,119,415, 3,994,704, 3,915,676, 3,513,635, and 2,307,602, Japanese documents JP363283768, JP362097650, JP362002844, and JP360129114, Soviet Union documents SU000929224 and SU00912218, and Netherlands document NL072006447. In general, the use of such electrostatic technology for particle control involves the use of electrostatic collectors to remove charged dust particles from an air stream. This is accomplished by subjecting the dust-laden air stream to an electric field to cause the dust particles therein to be charged. The charged, dust-laden air stream is then subjected to an oppositely-charged element wherein the charged dust particles are removed from the charged, dust-laden air stream.
A need exists to provide a system for collecting particulate debris generated during machining of articles, particularly post-cure machining of composite articles that utilizes aerodynamic and electrostatic forces in combination to provide high-efficiency collection of the particulate debris. The system should include a means for creating a localized, vacuum-induced aerodynamic fluid flow field in the article machining area. The system should also include a means for charging the particulate debris generated during machining of the article. Further, the system should include a means for creating an electric field that is collinear with the localized, vacuum-induced aerodynamic fluid flow field.